The present invention relates to transconductance amplifiers, and more particularly to a bidirectional chopper transconductance amplifier that works in four quadrants to provide drive signals for stepper and DC servo motors.
Stepper and DC servo motor drivers typically use a transconductance amplifier. A command voltage is input to the amplifier proportional to a desired motor winding current. The transconductance amplifier controls the voltage across the motor winding so that the current in the winding approximates the desired value. Since direct linear drive of the motor voltage uses excessive power, most transconductance amplifiers use a chopping technique. For example if ten volts is needed across the motor winding, it is switched between zero volts and the supply voltage, such as forty volts, with the appropriate duty cycle, such as 25%, to give an average voltage of ten volts. If minus ten volts is needed, then the opposite end of the motor winding is connected to the supply voltage with the same duty cycle. A set of four power switches, typically transistors or FETs, is used to connect either end of the motor winding to either the supply voltage or ground. This set of power switches is called an H bridge. As with a linear version, the motor winding current is monitored and the output voltage (duty cycle) is adjusted to cause the current to approximate the desired value.
A transconductance amplifier is said to work in four quadrants if the voltage across the motor winding may be either polarity independent of the motor winding current. This is useful where the command current is positive but decreasing rapidly. The voltage may need to be negative in order to decrease the current rapidly enough through the inductance of the motor winding. Efficient four quadrant operation is difficult to achieve, especially with standard H bridge integrated circuits that do not allow sensing the motor winding current when the output voltage is zero, the recirculate state. One method is implemented in the PHASER III Color Printer manufactured by Tektronix, Inc., Wilsonville, Oreg., United States of America. In that embodiment a small current sense resistor is placed in series with the motor winding so that the motor winding current is always being sensed, even when the H bridge is in the recirculate state. A precision differential amplifier converts a small voltage signal across the current sense resistor, representative of a relatively large current through the current sense resistor, to a ground referenced voltage that is proportional to the actual motor current. The commanded current is input as a ground referenced voltage that is the inverse of the desired current. The command voltage and the output of the precision differential amplifier are summed together with opposing offset voltages in two summers to produce opposing error voltages proportional to the difference between the desired current and the measured current. These error voltages are input to respective comparators together with a triangle wave signal whose peaks are less than the offset voltages. Depending upon the signs of the error voltages, one or the other of the comparators switches, causing the H bridge to either increase or decrease the motor current to correct the current error represented by the error voltages and rebalance the circuit.
What is desired is another method for a bidirectional chopper transconductance amplifier that does not require a precision differential amplifier or much analog circuitry.